I can tell you for a fact this is an NCH developer issue, not a problem with the driver. I ran into the same issue with my modem so I ran through NCH's troubleshooting tips and got no joy. Feeling adventurous, I tried duplicating the issue with a different product. I chose Venta Fax & Voice to troubleshoot. Wouldn't you know, my IVR functionality worked fine with Venta! The answering machine played through the modem properly instead of routing to the speakers. Venta is a vastly inferior product though, setting up something akin to IVM is a real PITA so I would prefer to stay with IVM.
I challenge NCH software support to try the same thing and see what I'm talking about. The modem I'm using is not on the preferred list, but it is 100% TAPI compliant and again, it works fine with Venta so the driver is set up correctly and works as expected provided the software on the other end is programmed correctly. The modem I've got is even a USB modem! It's an Hiro H50113. Here's the specs. It's even Vista and Windows 7 compliant!
So yeah, NCH needs to stop pointing fingers, turn that hand around and point to themselves.